This application claims the benefit of Korean Patent Application No. 1999-68050, filed on Dec. 31, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates an X-ray imaging device. More particularly it relates to an X-ray imaging device in which residual charges within a photo-sensing layer are rapidly removed.
2. Discussion of the Related Art
Imaging systems that employ X-rays have been successfully used in medical, scientific and industrial applications. One type of X-ray imaging device uses a photosensitive array panel comprised of a plurality of photo-sensing cells arranged in a matrix. Those cells sense irradiating X-rays that have passed through an object being imaged by generating electric charges in proportion to the intensity of the irradiating X-rays. The electric charges from the photo-sensing cells are sent to a signal converter that converts those charges into electrical signals, that are in turn sent to an image output device. The image output device processes the electrical signals to produce a screen display of the intensity patterns of the X-rays that irradiate the photosensitive array panel.
FIG. 1A schematically illustrates a sectional view of a photo-sensing cell, while FIG. 1B schematically illustrates a planar view of part of a photosensitive array panel. Referring now to FIG. 1A, the photo-sensing cell includes a gate line 22, a thin film transistor (TFT) 24, and a charging capacitor (Cst) that are formed on a glass substrate 20. A pixel electrode 32 electrically connects to a drain electrode 26 of the TFT and to the charging capacitor Cst. A photo-sensing layer 34 is formed on the pixel electrode 32. A dielectric insulating layer 36 is formed on the photo-sensing layer 38, and a conductive upper electrode 38 is formed on the insulating layer 36.
The photo-sensing layer 34 is photoconductive and is used to convert X-rays into electric charges. It is beneficially formed from amorphous selenium having a thickness of hundreds of micrometers.
As shown in FIG. 1A and FIG. 1B, the TFT 24 includes a gate electrode 30 that electrically connects to the gate line 22. Thus, control signals can be applied to the TFT via the gate line 22. The TFT 24 also includes a source electrode 28 that electrically connects to a data line 40 (see FIG. 1B) that is formed on the photo sensitive cell array panel in a direction perpendicular to the gate line 22. The drain electrode 26 electrically connects to the pixel electrode 32.
The pixel electrode 32 is formed within the photo-sensing cell to have an area as large as possible. This enables efficient collection of the electric charges generated in the photo-sensing layer 34 so that they can be stored in the charging capacitor Cst. A high voltage generator 42 connects to the upper electrode 38. That generator supplies a high voltage such that a strong electric field is produced in the photo-sensing layer 34.
In operation, X-rays pass through an object and then irradiate the photo-sensing layer 34. The irradiating X-rays (photons) produce electron-hole pairs within the photo-sensing layer 34. The high voltage (several tens of kV) from the high voltage generator 42 is applied to the upper electrode 38. That high voltage produces an electric field in the photo-sensing layer 34 that causes the electron-hole pairs to separate. The holes are collected by the pixel electrode 32 and then stored in the charging capacitor Cst. The TFT 24 serves as a switch that controls the outflow of stored electric charges in the charging capacitor Cst. When a gate voltage is applied, via the gate line 22, to the gate electrode 30 of the TFT 24, a channel is defined between the source electrode 28 and the drain electrode 26. Electrons stored in the charging capacitor Cst can then pass through the drain electrode 26 and through the source electrode 28 to the data line 40.
Referring now to FIG. 2, an X-ray imaging system includes a driving apparatus that converts the electric charges from the charging capacitors Cst into electrical signals that are applied to an output. The driving apparatus includes a photosensitive array panel 60 having photo-sensing cells 62 that are arranged in a matrix. A gate driver 64 connects to gate lines GL1 through GLm, and a data reader 66 connects to data lines DL1 through DLn. An output 68 displays electrical signals from the data reader 66 as an image.
In the photo sensitive array panel 60, the photo-sensing cells 62 are positioned near intersections of the m gate lines GL1 through GLm and the n data lines DL1 through DLn. Still referring to FIG. 2, each of the photo-sensing cells 62 includes a photo sensor 70, a charging capacitor Cst, and a TFT 72. In each photo-sensing cell 62, a gate electrode 74 of the TFT 72 electrically connects to one of the gate lines GL1 through GLm, and thus to the gate driver 64. Additionally, a source electrode 76 thereof electrically connects to one of the data lines DL1 through DLn, and thus to the data reader 66. Furthermore, a drain electrode of each TFT 72 is connected to a charging capacitor Cst.
When a gate control signal from the gate driver 64 is applied to one of the gate lines GL1 through GLm, the gate electrodes 74 connected to that gate line turn on their associated TFTs 72. Then, a conductive channel is defined between the drain electrode 78 and the source electrode 76 of each of the ON TFTs 72. Thus, the electric charge stored in the charging capacitor Cst of each ON TFT is transferred via one of the data lines DL1 through DLn to the data reader 66. In practice, the gate driver 64 applies a pulse-shaped gate control signal sequentially to each of the m gate lines GL1 through GLm. Thus, the stored electric charges are all applied to the data reader 66 in scan lines.
The data reader 66 generates electrical data signals that correspond to the electric charges from the photo sensitive array panel 60. The data reader 66 sequentially applies groups of n data signals that correspond to the intensity of the X-rays irradiated onto the photo sensitive cell array panel 60, plus a reference signal, to the output 68. The output 68 includes a differential amplifier and an analog-to-digital converter (not shown). The data signals applied to the output 68 are analog signals having noise. The output 68 differentially amplifies the data signals with the reference signal to remove that noise, and then converts the noise-removed analog signal into a digital signal that is suitable for producing an image output on a screen.
As previously mentioned X-rays that irradiate the photo sensitive array panel 60 produce electron-hole pairs in the photo-sensing layer 34. The high voltage from the high voltage generator produces an intense electric field that separates the electron-hole pairs. Thus, as shown in FIG. 1A, holes are collected by the pixel electrode 32 and stored in the charging capacitor Cst. However, the separated electrons accumulate in a region near the boundary between the insulating layer 36 and the photo-sensing layer 34. Such electrons are referred to as xe2x80x9cresidual charges.xe2x80x9d The residual charges do not simply disappear, they remain even after the electric charges stored in the charging capacitor Cst have been applied to the data reader 66.
The residual charges have an impact on charges stored in subsequent X-ray irradiations, and thus on subsequent electrical signals. A prior art approach to dealing with residual charges is to irradiate visible light onto the photo sensitive array panel after X-ray irradiation. For example, U.S. Pat. No. 5,563,421 discloses turning off the high voltage applied to the upper electrode 38 after X-ray irradiation and then irradiating the panel with visible light. The visible light produces new electron-hole pairs in the photo-sensing layer 34. The holes tend to re-combine with the residual charges, thus dissipating them. However, this approach has a drawback in that the visible light should be irradiated for tens of seconds, too long a time for many applications, such as producing moving picture displays.
Accordingly, the present invention is directed to an x-ray imaging device, and to a method of driving that x-ray imaging device, that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an X-ray imaging device and a driving method thereof wherein residual charges are rapidly removed, beneficially fast enough to produce a moving display.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an X-ray imaging device according to one aspect of the present invention includes a system for discharging residual charges into a ground voltage source.
An X-ray imaging device according to another aspect of the present invention includes an erasure electrode between a dielectric layer and a photo-sensing layer that is for accumulating residual charge. The X-ray imaging device further includes a switch that connects the erasure electrode to a ground voltage source. That switch is for controlling the discharge of the accumulated residual charges.
A method of driving an X-ray imaging device according to the principles of the present invention includes discharging residual charges separated from the pixel charges into a ground voltage source. Beneficially, this is performed after the stored charges in cell capacitors have been processed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.